The invention relates to a filling level measuring device operating with microwaves, having a metallic housing portion through which microwaves are transmitted and/or received and in which an insert composed of a dielectric is arranged. Furthermore, the invention relates to a process for producing the dielectric.
In filling level measurement, microwaves are sent by means of an antenna to the surface of a filled substance and the echo waves reflected at the surface. are received. An echo function representing the echo amplitudes as a function of the distance is formed and used to determine the probable useful echo and its delay time. The delay time is used to determine the distance between the surface of the filled substance and the antenna.
All known methods which make it possible to measure relatively short distances by means of reflected microwaves can be used. The most well-known examples are pulsed radar and frequency-modulation continuous-wave radar (FMCW radar).
In the case of pulsed radar, short microwave transmission pulses referred to in the following as wave packets, are transmitted periodically, reflected by the surface of the filled substance and received again after a distance-dependent delay time. The received signal amplitude as a function of time represents the echo function. Each value of this echo function corresponds to the amplitude of an echo reflected at a particular distance from the antenna.
In the case of the FMCW method, a continuous microwave which is periodically frequency-modulated linearly, for example on the basis of a sawtooth function, is transmitted. The frequency of the received echo signal therefore has with respect to the instantaneous frequency which the transmitted signal has at the instant of reception a frequency difference which depends on the delay time of the echo signal. The frequency difference between transmitted signal and received signal, which can be obtained by mixing the two signals and evaluation of the Fourier spectrum of the mixed signal, consequently corresponds to the distance of the reflecting surface from the antenna. Furthermore, the amplitudes of the spectral lines of the frequency spectrum obtained by Fourier transformation correspond to the echo amplitudes. Therefore, in this case, this Fourier spectrum represents the echo function.
Filling level measuring devices operating with microwaves are used in very many branches of industry, for example in chemistry or in the food industry. Typically, the filling level in a container is to be measured. These containers generally have an opening, at which a connection piece or a flange is provided for the fastening of measuring devices.
In industrial measuring technology, dielectric rod antennas and horn antennas are regularly used for transmitting and/or receiving. Typically, a pot-like housing which has the geometry of a short-circuited waveguide is used. An exciter pin, via which microwaves are transmitted and/or received through the housing, is inserted into said housing. In the case of a horn antenna, the housing is adjoined by a funnel-shaped portion which opens out in the direction facing the container and forms the horn. In the case of the rod antenna, a rod composed of a dielectric and pointing into the container is provided. The interior space of the housing is usually filled virtually completely by an insert composed of a dielectric. In the case of the horn antenna, the insert has a conical end, pointing into the container. In the case of rod antennas, the insert is adjoined by the rod-shaped antenna.
On account of the dimensioning of the waveguide and the dielectric constant of the insert, only certain modes can be propagated. For filling level measurements, modes having a radiation characteristic with a pronounced forward lobe are preferred, in the case of circular waveguides, for example, the transverse electric (TE-11) mode. The transmission frequency is also prescribed in most countries.
In order that the dimensions of the housing are nevertheless variable within certain limits, for example to perform adaptations to dimensions of containers, a dielectric with a substantially continuously adjustable dielectric constant is advantageous. In the following text, dielectric constant always refers to the dielectric constant which is based on the vacuum dielectric constant and the value of which is equal to the quotient of the dielectric constant divided by the vacuum dielectric constant.
In DE-A 44 05 855 there is described a filling level measuring device operating with microwaves
having a metallic housing portion,
through which microwaves are transmitted and/or received and
in which an insert composed of a dielectric is arranged.
It has a rod antenna and the insert and rod antenna are composed of a dielectric. It is specified to use plastic, glass or ceramic or a mixture of said materials for this purpose.
Insert can come into contact with a medium located in the container. Depending on the application, this may well be an aggressive medium. Consequently, the insert should have in addition to the mechanical resistance required for industrial applications also a high chemical resistance.
In the case of commercially available filling level measuring devices operating with microwaves, polytetrafluoroethylene (PTFE), which has a high chemical resistance, is often used for this reason. The dielectric constant of polytetrafluoroethylene (PTFE) is not variable, however.
In U.S. Pat. No. 5,227,749 microwave circuits and components are described, for example microwave striplines, in which desired electrical and mechanical properties are achieved simultaneously by using an enclosure filled with a dielectric. The enclosure offers adequate mechanical stability, so that the dielectric can be selected purely on the basis of its dielectric properties.
Although such a construction represents a feasible approach in the case of microwave striplines and microwave circuits, it is unsuitable however for use in an antenna. The housing and insert act as a waveguide in which the microwaves form. An enclosure would have different dielectric properties than the dielectric embedded in it and, owing to its nonisotropic electrical properties, would consequently lead to considerable disturbances in the desired modes during transmitting and/or receiving.
U.S. Pat. No. 4,335,180 there is described a dielectric for microwave circuit boards and a method of making it.
The dielectric consists of polytetrafluoroethylene (PTFE), a filler material and a fibrous material. The proportion of filler material is specified as 10 to 75 percent by weight. Among the materials specified as, the filler material is aluminum oxide. The proportion of fibers is between 2.5 and 7 percent by weight of the dielectric and ensures its mechanical stability. The dielectric constant of the material is specified as 10 to 11.
This dielectric is made by blending the filler material and fibrous material into a polymer dispersion. A flocculant is added to the slurry thus formed until a dough-like material is produced, which is then shaped and dried.
In a circuit board, the fibers can be aligned in a plane by appropriate processing, for example pressing or rolling, so that a substantially homogeneous thin sheet, that is a substantially two-dimensional formation, is produced. A three-dimensional body cannot be readily produced in this way, however. In a three-dimensional body, fibers cannot be aligned in one plane by pressing or rolling. Raised fibers behave like small quills and the body remains correspondingly porous and inhomogeneous in spite of pressing. It would consequently have less mechanical strength and inhomogeneities would lead to reflections of the microwaves. There is also the risk of the porous material being saturated with moisture. Moisture in the material leads to a high loss factor tan xcex4.
It is an object of the invention to specify a filling level measuring device operating with microwaves, having a housing and an insert composed of a dielectric, and a process for producing the dielectric, in which the dielectric constant of the insert is adjustable and in which the insert has a high chemical resistance and a mechanical strength adequate for industrial applications.
For this purpose, the invention comprises a filling level measuring device operating with microwaves
having a metallic housing portion,
through which microwaves are transmitted and/or received and
in which there is arranged an insert composed of a dielectric, which consists of a composite material composed of a fluoroplastic, in particular polytetrafluoroethylene (PTFE), and ceramic.
According to an advantageous refinement, the composite material has a proportion of ceramic which is below the percolation limit.
According to a further refinement, the composite material has a dielectric constant xcex5 and the quotient of the dielectric constant xcex5 and the vacuum dielectric constant xcex50 has a value between 2 and 10. Furthermore, the composite material preferably has a loss factor tan xcex4 which is less than one fiftieth.
According to an advantageous development, the insert has in a portion of the housing arranged in the direction of transmission a lower proportion of ceramic than in a portion facing away from the direction of transmission.
Furthermore, the invention includes a process for producing a composite material from fluoroplastic, in particular polytetrafluoroethylene (PTFE), and ceramic, which comprises the following steps:
a) producing a mixture of powdered ceramic and powdered fluoroplastic,
b) drying the mixture,
c) pressing the mixture and
d) sintering the pressed mixture.
According to an advantageous refinement of the process, the proportion of ceramic in the mixture is below the percolation limit.
According to a further refinement of the process, the quotient of the dielectric constant xcex5 of the composite material and the vacuum dielectric constant xcex50 has a value between 2 and 10 and the composite material has a loss factor tan xcex4 which is less than one fiftieth.
According to a further refinement of the process, two or more mixtures with different proportions of ceramic are produced, and the mixtures are layered one on top of the other before pressing in such a way that the proportion of ceramic in the composite material decreases from layer to layer.
The invention and further advantages are now explained in more detail with reference to the figures of the drawing, in which two exemplary embodiments of a filling level measuring device operating with microwaves are represented; identical parts are provided in the figures with identical reference numbers.